linux-kernel - Re: [patch 9/9] mm: keep page cache radix tree nodes in check

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Message-ID: <20140122184836.GE4407@cmpxchg.org>
Date:	Wed, 22 Jan 2014 13:48:36 -0500
From:	Johannes Weiner <hannes@...xchg.org>
To:	Dave Chinner <david@...morbit.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Andi Kleen <andi@...stfloor.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Bob Liu <bob.liu@...cle.com>,
	Christoph Hellwig <hch@...radead.org>,
	Greg Thelen <gthelen@...gle.com>,
	Hugh Dickins <hughd@...gle.com>, Jan Kara <jack@...e.cz>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Luigi Semenzato <semenzato@...gle.com>,
	Mel Gorman <mgorman@...e.de>,
	Metin Doslu <metin@...usdata.com>,
	Michel Lespinasse <walken@...gle.com>,
	Minchan Kim <minchan.kim@...il.com>,
	Ozgun Erdogan <ozgun@...usdata.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Rik van Riel <riel@...hat.com>,
	Roman Gushchin <klamm@...dex-team.ru>,
	Ryan Mallon <rmallon@...il.com>, Tejun Heo <tj@...nel.org>,
	Vlastimil Babka <vbabka@...e.cz>, linux-mm@...ck.org,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [patch 9/9] mm: keep page cache radix tree nodes in check

On Wed, Jan 22, 2014 at 01:57:14AM -0500, Johannes Weiner wrote:
> Not at this time, I'll try to look into that.  For now, I am updating
> the patch to revert the shrinker back to DEFAULT_SEEKS and change the
> object count to only include objects above a certain threshold, which
> assumes a worst-case population of 4 in 64 slots.  It's not perfect,
> but neither was the seeks magic, and it's easier to reason about what
> it's actually doing.

Ah, the quality of 2am submissions...  8 out of 64 of course.

> @@ -266,14 +269,38 @@ struct list_lru workingset_shadow_nodes;
>  static unsigned long count_shadow_nodes(struct shrinker *shrinker,
>  					struct shrink_control *sc)
>  {
> -	return list_lru_count_node(&workingset_shadow_nodes, sc->nid);
> +	unsigned long shadow_nodes;
> +	unsigned long max_nodes;
> +	unsigned long pages;
> +
> +	shadow_nodes = list_lru_count_node(&workingset_shadow_nodes, sc->nid);
> +	pages = node_present_pages(sc->nid);
> +	/*
> +	 * Active cache pages are limited to 50% of memory, and shadow
> +	 * entries that represent a refault distance bigger than that
> +	 * do not have any effect.  Limit the number of shadow nodes
> +	 * such that shadow entries do not exceed the number of active
> +	 * cache pages, assuming a worst-case node population density
> +	 * of 1/16th on average.

1/8th.  The actual code is consistent:

> +	 * On 64-bit with 7 radix_tree_nodes per page and 64 slots
> +	 * each, this will reclaim shadow entries when they consume
> +	 * ~2% of available memory:
> +	 *
> +	 * PAGE_SIZE / radix_tree_nodes / node_entries / PAGE_SIZE
> +	 */
> +	max_nodes = pages >> (1 + RADIX_TREE_MAP_SHIFT - 3);
> +
> +	if (shadow_nodes <= max_nodes)
> +		return 0;
> +
> +	return shadow_nodes - max_nodes;
>  }
>  
>  static enum lru_status shadow_lru_isolate(struct list_head *item,
>  					  spinlock_t *lru_lock,
>  					  void *arg)
>  {
> -	unsigned long *nr_reclaimed = arg;
>  	struct address_space *mapping;
>  	struct radix_tree_node *node;
>  	unsigned int i;
--
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